This invention concerns a light emitting element intended for use in a matrix display board.
Up to the present time two types of elements have been proposed and employed for use in display boards: incandescent lamps and cathode ray tubes.
The main difficulties of incandescent lamps reside in their high power consumption (between 20 and 40 watts per element) and their relatively low yield (about 10 lumens per watt). It will also be noted that such lamps are relatively short-lived (average 1000 hours) and have a colour temperature which is variable as a function of the energizing voltage at the terminals thereof as well as a progressive diminution of the luminescent intensity because of the interior blackening of the bulb as a function of the length of service. Such elements have likewise been proposed to equip display boards in colour. In such cases for each pixel three lamps are required associated with coloured filters, or more simply, three lamps each having a coloured bulb. It will be however understood that for colour the difficulties mentioned above in respect of black and white displays are entirely present. On the positive side however it may be said that incandescent lamps are inexpensive elements easily changed and readily to be found on the market.
Thus, in a practical example, a black and white screen of 4.3 m high and 8.6 m wide comprises 160 lines and 80 columns, thus requiring 12,800 incandescent lamps. If the rating of each lamp is 25 W, the power necessary to energize all of them simultaneously to full luminosity will be on the order of 320 kW. It will be readily understood that such as screen requires a high capacity power source as well as a very considerable energy expense.
Cathode ray tubes have been used in colour screens as may be seen in the British patent publication No. 2 053 547 and U.S. Pat. No. 4,326,150. Although these concern a tube of which the manufacture is simplified relative to those known in television tubes, it is nonetheless quite complex and above all necessary to employ very high acceleration voltages therewith, this complicating considerably the realization of the assembly. Such a tube has however the advantage of a low energy consumption compared to that of an incandescent lamp.
To overcome the difficulties mentioned above, the present invention proposes a light emitting element comprising at least one discharge tube containing mercury vapour at low pressure the internal wall of said tube being coated with a fluorescent substance.
Such an element is known of itself but seems not to have been previously proposed for use in a matrix display board. In most cases it is applied in domestic lighting or for luminous signs.
In the first case, variable length tubes which may be straight or curved, have at each end electrodes which may be constituted by a filament coated with an emitting layer of oxyde. The gaseous atmosphere within the tube is comprised of argon for start-up at a pressure of several millimeters of mercury and of a drop of mercury. The discharge in the mercury vapour effects essentially ultra-violet radiation at a wavelength of 253.7 nm. The wall of the tube appears white from the nature of the fluorescent substance (phosphor) applied on the interior wall and intended to convert the ultra-violet radiation into visible light.
Certain luminous signs use a type of tube known as luminescent for which the discharge in the gas creates directly the luminous effect. In this case, the wall of the tube is either transparent or coloured without employing however the fluorescent phenomenon. The arrangement described in British specification No. 354 908 makes use of tubes filled with neon which gives an orange-red colour or mercury vapour which gives a blue colour. However the arrangement shown does not in any way constitute a matrix display since it comprises a multitude of rectilinear segments of various lengths and interlaced in such a manner as to form a letter or number by the illumination of a predetermined number of the segments. The light emitting element described in the German specification No. 2 031 610 also makes use of neon tubes to set up a display system for moving script. The cited element comprises three tubes emitting different colours. However no means are shown for mixing the colours so as to permit the obtaining at the element output light the resultant wavelength of which may vary throughout the visible spectrum. Generally luminescent tubes are poorly adapted to use in a matrix display since in order to obtain the three fundamental colours one is obliged to combine the filling gas with the colour of the tube, this leading to elements which will not emit the same light intensity for each of the three tubes.
With respect to the incandescent lamp, the fluorescent tube presents several advantages. It has a high yield of light on the order of 40 lumens per watt, this resulting for a comparable luminous flux in a considerably diminished energy consumption. The average life span exceeds 7,500 hours, this contributing to increase the reliability of the entire display. It likewise displays a very greatly diminished heat output this having as effect to reduce convection currents and blackened trails of dust brought about by such convection. Finally the tube displays a colour temperature which is invariable as a function of the luminosity by as well as a very feeble blackening of the bulb, localised at the placing of the electrodes, as a function of its length of service.
Relative to the cathode ray tube, the fluorescent tube shows energy consumption approximately the same. On the other hand, its price is considerably lower and it does not require to be energized at a high voltage. Finally, the number of electrodes is reduced.
Thus, the use of a fluorescent tube in a giant display screen as foreseen in the present invention enables the offering of a new and advantageous product by virtue of its lower energy consumption, the quality of the images presented and its reasonable price.